Nitrocellulose-based propellant compositions are well known in the art, having wide ranging utility in the military, aerospace and civilian industries. For example, such propellant compositions are used as smokeless explosive charges for artillery and small arms, for solid fuel rocket engines and in blasting compositions employed within the construction industry.
Conventional granular, nitrocellulose-based propellant compositions generally contain nitrocotton (nitrocellulose), selected organic or inorganic salts for use as ballistic modifiers or stabilizers, and other additives such as carbon black. If other energetic bases such as nitroguanidine or nitroglycerine are also added, the propellant is termed a "multiple base" propellant. Thus, increasing the number of energetic bases within the propellant provides an effective means to enhance muzzle velocity of the charge and thereby increase shooting performance. Despite wide acceptance, conventional nitrocellulose-based propellants suffer from susceptibility to degradation if subjected to high humidity or water immersion. Conventional nitrocellulose-based propellants require careful storage and handling procedures in order to avoid accidental contact with moisture.
Prior art attempts have been made to waterproof conventional nitrocellulose-based propellants, however these have proven ineffective. Previous methods of waterproofing have concentrated on means to coat or encapsulate the individual propellant grains. This approach has resulted in either a reduction in performance of the explosive, an increase in residue and carcinogens upon ignition or less than favorable water resistance.
A further problem connected with conventional nitrocellulose-based propellants arises when attempting to produce a caseless charge from such propellants. Generally, a caseless charge must be designed so that, upon ignition, burning will not be limited to the surface of the charge but will occur throughout the cross-section of the charge as is found in conventional charges held by casings. In one prior art method, caseless cartridges have been made by compressing the individual propellant grains followed by solvent dipping or coating of the exterior of the cartridge to harden its surface. Cartridges produced by this method have been found to have suitable surface strength but lack overall strength and frequent breakages still occur. Further, this prior art method requires that the degree of compaction be sufficient to bind the individual grains so as to prevent breakage during normal handling yet not so great as to interfere with the friability of the individual grains thereby allowing each grain to burn separately and uniformly as if in a loose charge.
Another approach to the manufacture of caseless charges involves contacting the propellant grains with an aqueous solvating solution. Cartridges produced by this method are generally found to be too weak to withstand the normal handling required of ammunition. This is particularly true when such caseless charges are employed in bazookas, an armament requiring wafer-thin charges.
A still further problem associated with conventional nitrocellulose-based propellants is the limitation imposed upon such propellants when the various energetics chosen to be included within the propellant are antagonistic toward each other. For example, nitroglycerine, picric acid, nitroguanidine, cyclotetraethylenetetranitramine (HMX) and cyclotrimethylenetrinitramine (RDX) are all explosive compounds having varying performances, compatibilities, physical properties and sensitivities. Intermixing these various energetic compounds within a single multiple-base propellant does have limitations in that each of the components possess separate impact and interaction sensitivities. As a result, the potential liabilities of combining such highly volatile and explosive components often outweigh the inherent benefit of heightened shooting performance.
Prior art nitrocellulose based propellants also suffer from problems with respect to their temperature-dependent physical properties once they are molded into a caseless form. For example, a desired characteristic of a solid propellant is that it provide use over a fairly wide range of temperatures yet maintain its impetus. A solid propellant should also be flexible enough at lower temperatures to withstand rough handling and firing without fracturing of the grain structure. At elevated temperatures, the propellant must have sufficient firmness so that it will not melt, flow or migrate prior to use. These requirements are particularly apparent when considering the different physical environments into which such propellants are used; from arctic to jungle and desert locales.
Prior art nitrocellulose-based propellant compositions do not presently meet these temperature requirements and especially at lower temperatures. Attempts to remedy the problem have focused on increasing the amount of plasticizer within the propellant composition. Although such additions render the nitrocellulose-based propellant more flexible at low temperatures, there still exists an upper limit on the amount of plasticizer which can be incorporated. Beyond that point, the mixture tends not to cure into a solid. Further, excessive plasticizer has been known to separate within or otherwise externally bleed from the propellant thereby rendering the composition useless or even dangerous.